Hydrogen Evolution during the Corrosion of Galvanically Coupled Magnesium

Abstract

In this study, the impact of galvanic coupling of magnesium to steel on the corrosion rate, surface morphology, and surface film formation was investigated. In particular, experiments were performed to examine and quantify the role of self-corrosion (also called negative difference effect (NDE) or anodic hydrogen) during the corrosion of galvanically coupled Mg. It was found that galvanic coupling at high cathode-to-anode area ratios resulted in high rates of corrosion that impacted hydrogen evolution on the Mg surface. Self-corrosion accounted for, on average, approximately one-third of the total observed corrosion. The self-corrosion fraction varied with time and was found to reach values in excess of 50%. Surface film formation was observed, and approximately 30% of the Mg lost to corrosion was found in the film at the end of our experiments. The surface morphology observed during galvanic corrosion was dramatically different from the filiform structures associated with free corrosion of our samples, but showed similarities to the morphology observed previously for anodically polarized samples. Film formation appeared to slow the rate of self-corrosion with time. These results complement previous studies of Mg corrosion and add important insight into the role of hydrogen evolution on the Mg surface during galvanic corrosion. New environmental regulations, good mechanical properties, abundant availability and reasonable cost have generated renewed interest in the wider use of magnesium (Mg), especially for automobile and aerospace applications.1-4 There has also been growing interest in the use of Mg for batteries and in hydrogen generation systems. 5-8A key obstacle to the use of Mg is its corrosion susceptibility. Consequently, several previous studies have focused on understanding the corrosion mechanisms that control the dissolution rate and surface morphology of Mg during corrosion. Both disk-shaped and filiform corrosion have been observed for unalloyed Mg, depending on the concentration of impurities in the metal and the concentration of Cl − in the electrolyte. [9][10][11] In both cases, the morphology is strongly influenced by the cathodic reaction, which limits the rate of corrosion. 11,12The situation changes dramatically when Mg is coupled to another metal because of the increased cathodic area that significantly diminishes limitations due to the cathodic reaction on the Mg surface. Although most hydrogen evolution occurs on the coupled metal, hydrogen evolution on the Mg surface does not cease. The purpose of this paper is to investigate the impact of galvanic coupling on Mg corrosion that is associated with hydrogen evolution on the Mg surface, and the extent to which such coupling influences both the morphology and rate of Mg self-corrosion.In this paper, we use the term "self-corrosion" to refer to the dissolution of Mg that is not associated with the galvanic current that passes between the coupled metal and the Mg, similar to the definition used previously by others in galvanically coupled st...

The inhibition effect of sodium tetraborate on the corrosion of steel in KOH media

“…Initially the corrosion rate is fast, but with increasing amount of added Na 2 B 4 O 7 evolved H 2 decreases, that is, the corrosion rate decreases. These results are in conformity with the literature results [32,33].…”

Ethanol as an electrolyte additive for alkaline zinc-air flow batteries

“…The hydrogen evolution observed in cathodic branches was enhanced for the electrolytes containing ethanol. It was reported the effects of hydrogen absorbed on metal surfaces on anodic polarization in several aspects including retarding the formation of passivating film, changing the condition of electrode surface, decreasing the resistance toward charge transfer and ion diffusion, and increasing the capacitance 37 , 38 .…”

Accelerating Corrosion of Pure Magnesium Co-implanted with Titanium in Vivo

“…In general, the corrosion of magnesium includes self-corrosion and galvanic corrosion. It was reported that during galvanic corrosion, the surface morphology was dramatically different from the filiform structures associated with free corrosion 30 . In this study, the SEM demonstrated the change of surface morphology, indicating the galvanic-like corrosion.…”